This invention relates to the construction of an extrusion die for extruding flowable materials into a honeycomb structure for use in an automotive catalytic converter and other honeycomb structures, such as particulate traps for diesel engines.
Extrusion dies have been found to be useful in forming cellular or honeycomb ceramic substrates for use as catalytic converters utilized in the exhaust system of internal combustion engines or particulate traps for diesel engines. In order for such converters to function efficiently, it is necessary that the cells provide a substantially large surface area for catalytic material to react with the exhaust gases, and that the cell walls have a substantially thin cross-sectional dimension so as to provide a substantially large open frontal area, thereby reducing back pressure within the entire exhaust system. However, the thin-walled structure must simultaneously exhibit sufficient mechanical and thermal integrity so as to withstand normal automotive impact and thermal requirements.
These honeycomb structures typically have transverse cross-sectional cellular densities of approximately fifty (50) to one hundred fifty (150) cells or more per square centimeter. The reference numeral 10 (FIG. 1) generally designates a catalyst support body subsequent to extrusion and that is generally well known and that includes a honeycomb structure 12 formed by a matrix of intersecting, thin, porous walls 14 surrounded by an outer wall 16, which in the illustrated example is provided in a circular cross-sectional configuration. The walls 14 extend across and between a first end face 18 and an opposing second end face 20, and form a large number of adjoining hollow passages or cells 22 which also extend between and are open on the end faces 18, 20 of the filter body 10. After being coated with a catalyst, honeycomb structure 12 may be used in an automotive catalytic converter. In the formation of a particulate trap 11 (shown in FIGS. 2 and 3), one end of each of the cells 22 is sealed, a first subset 24 of the cells 22 being sealed at the first end face 18, and a second subset 26 of the cells 22 being sealed at the second end face 20 of the filter 11. Either of the end faces 18, 20 may be used as the inlet face of the resulting filter 11.
Extrusion dies of various configurations have been utilized to produce these honeycomb structures, including those which are disclosed in Yamamoto, U.S. Pat. No. 4,354,820; Reed, U.S. Pat. No. 4,235,583; and, Bagley, U.S. Pat. No. 3,905,743.
A significant drawback of certain prior art extrusion dies may be seen by reference to FIG. 1 of each of the Yamamoto and Reed patents. In both of these structures, the inlet portion of the die is provided with a plurality of cylindrical feed holes having downstream ends that terminate at the entrance or upstream portions of respective intersecting discharge slots, with alternate diagonal intersections of the discharge slots being directly fed by and aligned with the feed holes in the die. As disclosed by Yamamoto, the material being extruded enters into the feed holes each denoted by the numeral 2. The lower end of each of the feed holes is interrupted by tapered portions indicated by the numeral 20 in FIG. 3. After passing beyond the flow constriction, the extruded material flows into the outlet or discharge end of the die, for final extrusion through the discharge slots. As a result of the configuration disclosed by Yamamoto, there is an abrupt change in the cross-sectional area and shape of flow at the lower ends of the cylindrical feed holes, such change caused by the tapered portions 20. Within Reed, a somewhat similar construction is shown wherein the exit portions of cylindrical feed holes 7 are abruptly narrowed down at the entrance to the discharge slots, the latter being denoted by the numeral 9.
The extrusion dies as disclosed by Yamamoto and Reed each include four over-hanging, flow constricting portions at the outlet end of each feed hole, resulting in an abrupt decrease in the cross-sectional area of the feed holes. As a result, the die is subjected to a high stress due to the back pressure which develops from the extruded material abutting or flowing against each area overhanging the feed holes. In turn, this requires either the use of die materials having greater strength, or results in a limitation to the feed hole density of dies formed from materials having the greatest strength. Moreover, in the event that the material being extruded contains abrasive material, dies configured in such a manner are subject to greater wear and hence increased die degradation.
Other configurations of extrusion dies include monolithic billet dies, such as shown in FIGS. 1–6 of Bagley, wherein the die is formed in a unitary die block by utilizing conventional machining and cutting techniques, electric discharge machining, or electrochemical machining. Generally, unitary die blocks are formed of metals that not only facilitate ease of machining, but also provide a degree of elasticity to accommodate stresses and bending moments generated centrally of the discharge face during the application of high extrusion pressures.
As further disclosed in FIGS. 7, 8 and 9 of the Bagley patent, extrusion dies for honeycomb ceramic substrates may also be formed from a plurality of elongated extrusion plates which are clamped together in a stacked condition to form a laminated extrusion die. A major advantage of this configuration is the fact that each plate may be formed of an extremely hard wear-resistant material. However, like all of the other techniques disclosed above, such a configuration requires precise machining of slots and drilling of small holes to make such configuration. As a result of the complex machining required, such configurations have not yet been commercialized.